Charged particle beam diffuser



July 21, 1959 s, R, FARRELL 2,896,103

CHARGED PARTICLE BEAM DIFFUSER Filed Nov. 17, 1958 s Sheets-Sheet 1 @7 x 7 mm mm I Sherman R. Fa'rrell IE' I G1 2 BY mama H.

ATTORNEY INVENTOR.

uly 21, 1 s. R. FARR-ELL I 2,896,103

CHARGED PARTICLE BEAM DIFFUSER Filed Nov. 17, 1958 3 Sheets-Sheet 2 1N VENTOR.

Sherman R. Farrell ATTORNEY July 21, 1959 I s, FARRELL 2,896,103

CHARGED PARTICLE BEAM DIFFUSER Filed Nov. 17, 1958 3 Sheets-Sheet .3

i i l Sherman R. Farrell INVENTOR. a L b c e d A-QLQM, a BIG-L 5E3 ATTORNEY United States Patent CHARGED PARTICLE BEAM DIFFUSER Sherman R. Farrell, El Sobrante, Calif., assignor to Applied Radiation Corporation, Walnut Creek, Calif., a corporation of California Application November 17, 1958, Serial No. 774,415

6 Claims- (Cl. 3137) The present invention relates to means for distributing the intensity of an incident charged particle beam over the surface of material undergoing irradiation, and more particularly to a compact steady-state beam diffuser which is capable of spreading a charged particle beam over a relatively large area to produce an enlarged dosage area of material thereby irradiated.

In the irradiation of material with a beam of energetic charged particles it is oftentimes desirable to distribute the beam intensity over relatively large surface areas greater than the area intersected by the normal cross-sectional area of the incident beam. To accomplish this end, the charged particle beam is conventionally operated upon by suitable time varying magnetic deflection apparatus such as alternating current energized electro-magnets disposed on opposite sides of the beam. The deflection apparatus causes the beam to be alternately deflected back and forth across the material undergoing irradiation thus distributing the beam intensity over an elongated strip across the material. The material may then be moved transverse to the direction of beam deflection such that the entire surface area of the material is scanned by the beam. It is readily apparent that in order for the surface of the material to receive a relatively uniform dosage of irradiation, in the foregoing manner, the beam deflection or scanning rate must be appropriately synchronized with the speed of material movement. Conventional cyclic beam deflection apparatus thus depends upon relatively complex synchronizing apparatus for its successful operation. Such synchronizing apparatus is generally diflicult to adjust, diflicult to maintain in proper adjustment, and subject to break down by virtue of the electrical components necessarily employed therein. Cyclic deflection apparatus is thus rendered variously disadvantageous in the foregoing respects.

The disadvantages associated with cyclic beam scanning systems are overcome by the employment of steady-state heterogeneously distributed fields to continuously spread or diffuse a charged particle beam over large dosage areas of material undergoing irradiation. A steady state heterogeneous magnetic field wherein the spacing between the field lines progressively increases, and therefore the field strength decreases, in a direction lateral to the beam axis continuously operates to spread the beam over an increased cross-sectional area. Such beam enlargement is effected by virtue of the varied amounts of magnetic deflection imparted to the individual beam particles being dependent upon the lateral positions of such particles in the beam. More particularly, the charged particles laterally disposed in the beam in positions of progressively increasing field strength are deflected from the beam axis by correspondingly progressively greater amounts, thus enlarging the beam cross-sectional area laterally of the beam axis. It will be appreciated that in order for the charged particle beam to be distributed over large dosage areas, the variation in magnetic deflection forces relative to the lateral positions of the charged particles in the beam must be correspondingly large. Moreover, it is ice .made larger than heretofore possible with related apparatus of equivalent size. More particularly, the beam difluser of the instant invention utilizes to great advantage a secondary magnetic field to partially cancel a primary steady-state heterogeneous magnetic field generated transverse to the path of a charged particle beam. Such partial cancellation of the primary field provides an extremely large gradient of the field strength lateral to the beam axis and hence a correspondingly large variation in the deflection forces imparted to the beam particles. In addition spatial distributions of the primary and secondary fields are such that the distance over which the resultant field acts upon each beam particle, and therefore the amount of deflection imparted thereto, depends upon the lateral position of such particle in the beam. The foregoing two effects cooperate in the beam ditfuser of the present invention to materially enlarge a charged particle beam laterally of the axis thereof and thereby distribute the beam intensity over a large dosage area. Moreover, provision is made to adjust the strengths of the primary and secondary fields of the beam diffuser in order that the dosage area irradiated by the beam may be varied asdesired. The beam diffuser of the present invention thus steady-state beam diffusion apparatus over cyclic beam scanning devices as well as the additional advantages mentioned hereinbefore.

It is therefore an object of the present invention to provide a compact steady-state beam diffuser for producing a relatively large variation in beam particle deflection forces lateral to the beam axis.

It is another object of this invention to provide a charged particle beam diffuser for producing a readily adjustable amount of beam diffusion.

A further object of the invention is the provision of a rugged beam diffuser which requires a minimal amount of adjustment and maintenance for its reliable operation over long periods of service.

Additional objects and advantages of the invention will become apparent upon consideration of the following description in conjunction with the accompanying drawing, of which:

Figure 1 is a plan view of a preferred embodiment of the beam diffuser of the present invention,

Figure 2 is a cross-sectional elevation view of this embodiment taken along the line 2-2 of Figure 1,

Figure 3 is a plan view of this embodiment,

Figure 4 is a cross-sectional plan view of the embodirznent of Figure 1 taken along the line 44 of Fig ure Figure 5 is a schematic illustration of magnetic field s in accordance with the present invention as generated by the pole pieces of the preferred embodiment, portion (a) being a plan view of the fields and portion (b) being an elevational view thereof, with portion (b) dis posed in corresponding position beneath portion (1:).

Considering now the invention in some detail and referring to the illustrated form thereof in the drawing, there is provided a beam diffuser in accordance with the present invention which generally includes means for establishing an evacuated region adapted to receivea beam of energetic charged particles. Magnet means are provided to generate within the evacuated region a primary heterogeneous magnetic field transverse to the axis of beam propagation through such region with the strength of the field progressively increasing laterally outward from the beam axis. Moreover, the field is spatially distributed along the beam axis such that as the field strength progressively-increases laterally outward from the beam axis, the axial distance in the direction of beam motion over which the field is effective also progressively increases. More specifically; the regions'of greatest fieldstrength spaced relatively far from the beam bias act upon the beam particles'traversing such regions over relatively greater axial distances than the distances over which the regions of least intensity near theaxis act upon beam particles passing therethrough. Magnet means are also-provided to generate a secondary heterogeneous field of similar configuration as theprimary field within the evacuated region transverse to the beam axis. Such secondary field is generated in lateral spatial opposition to the primary field and is of opposite-magnetic polarity thereto such that the secondary field partially cancels theprimary field-and vice versa. The foregoing partial cancellation of magnetic fields produces a resultant relatively large gradient in field strength symmetrically acrossthe beam axis. Such large gradient in field strength together with the variation in axial distance over which the fields act as provided by the beam diffuser of the present invention are effective in transversely spreadinga charged particle beam over a larger area than heretofore possible with conventionalbeam difiusers of similar size.

As regards preferred structure for the beam diffuser of the present invention and referring particularly to Figures 1-4 of the drawings, it will be noted that the evacuated space of previous mention is best provided by means of an. elongated rectangular vacuum tank 11. Such-tank is. preferably provided with an inwardly stepped cylindrical inlet section.12 projecting coaxially from the upper end of the rectangular portion of the tank. The end of; inlet. section 12 is flanged as shown generally at 13 to facilitate vacuum tight coaxial end attachment to the beam exit window of a high. energy charged particle accelerator or other suitable source of energetic charged particles. A charged particle beam may thus be readily directed coaxially into inlet section 12 to pass axially through'the vacuum tank. In addition the lower end of vacuum tank 11 is preferably provided with a salient annular flange 14 to facilitate pressure sealed connection of a pumping manifold closuresection having a beam radiation window therein or equivalent means well known in the art, to facilitate pressure sealed connection to a vacuum pump while being transparent to an energetic chargedparticle beam.

In order to generate the-hereinbefore mentioned primary magnetic field in accordance with the present invention, appropriately shaped, transversely spaced-apart pole pieces 16, 17 are provided within vacuum tank 11 on one side of the axis thereof. More particularly, pole pieces 16, 17 are preferably secured in transverse opposition to opposite axial sides of vacuum tank 11 with the outer sides of the pole pieces in intimate contact with the sides of the tank. To accomplish the foregoing, the outer sides of pole pieces 16, 17 are advantageously provided with shoulders 18, 19 adapted to engage conformed recesses 21, 22 provided in the inner surfaces of the opposite axial sides of tank 11. In addition, spot faces 23, 24 are respectively provided in the inner side surfaces of pole pieces 16, 17 to engage the opposite ends of a compression spring 26 transversely inserted therebetween. The forces exerted by the compression spring 26 transversely outward thus maintain pole pieces 16, 17in intimate engagement with the opposite axial sides of the vacuum tank 11. The tips 27, 28 of pole pieces 16, 17 respectively are shaped to provide the previously noted spatial distribution of field along the vacuum tank axis such that the axial distance. over which the field acts is dependent upon lateral displacement from the axis. More particularly, the pole tips 27, 28 are preferably inclined toward the vacuum tank axis in the direction of beam motion therealong. Thus the lateral distance from the axis to a plane parallel to the axis and passing through the upper edges of pole tips 27, 28 respectively proximate the beam entry end of vacuum tank 11 exceeds the lateral distance to a parallel plane through the lower edges of the pole tips proximate the beam exit end of the tank. Thus the distance between the points of entry and departure of an axially moving charged particle in traversing the intervening space between pole tips 27, 28, and therefore the axial distance over which the particle is sub jected to the magnetic field generated therefrom, varies directly with respect to the lateral displacement of the particle from the axis of vacuum tank 11.

A suitable source of magnetomotive force is coupled between the pole pieces 16, 17 in order to establish the primary heterogeneous magnetic field in accordance with the present invention therebetween. The source of magnetomotive force may be, for example, a direct current electromagnet, or more preferably, as in the case of the preferred embodiment herein illustrated and described, a permanent horseshoe magnet 29. Such a horseshoe magnetmay 'bestbe disposed to partially externally encompass vacuum tank 11 in the mid-length region thereof with the opposite poles of the magnet in intimate contact with the opposite axial sides of the vacuum tank. Mounting of magnet 29 in the foregoing position may be advantageously facilitated by means of axially elongated mounting blocks 31, 32 rigidly secured to the sides of the vacuum tank and projecting laterally therefrom to provide flush abutments for engaging and securing the pole. tips of the magnet. Moreover, the mounting blocks 31, 32 extend axially from the mid-length region of the vacuurntank proximate the magnet pole tips to positions externally opposite pole pieces 16, 17 respectively in order to enhance magnetic coupling of the magnet to the pole pieces. Magnetic flux emanating from magnet 29, thus preferentially passes through one of the mounting blocks to the correspondingpole piece, then traverses the evacuated region within the vacuum tank to the opposing pole piece to thus establish the primary heterogeneous magnetic field, and thereafter returns through the other mounting block to the magnet? Although various means of attachment may be utilized to. secure magnet 29 to mounting blocks 31, 32, it is desirable that the means. employed render the magnet adjustably translatable with. respect to the mounting blocks. An air gap of adjustable length may then be established as desired between the pole tips of the magnet and the mounting blocks with an attendant variation in the field strength of the primary field generated in vacuum tank 11. Accordingly, preferred fastening means for securing magnet 29 to mounting blocks 31, 32 comprises jack bolts 33, 34 respectively threadably engaging the blocks, and jack screws.36,. 37. inserted through the pole tips of the magnet into. rigid engagement with jack bolts 33, 34. The jack screws 36, 37 may accordingly be rotated in one direction to tighten the magnet against the mountingblocks and in the'reverse direction to back off the magnet from the blocks and thusestablish the field strength varying air gap of adjustable length.

Considering now the secondary heterogeneous mag? netic field which is generated in the beam diffuser of thepresent invention, it is to be noted that such field is preferably provided by means similar to those employed to generate; theprirnary field. More particularly, trans? versely spaced-apart pole pieces 38, 39 are disposed on the opposite side of the axisof vacuum tank 11 from primary field pole pieces 16, 17 and with equal. lateral displacement thereat. The secondary pole pieces 38, 39 may be positioned directly opposite primary pole pieces 16, 17, or more preferably are axially staggered with respect thereto as depicted in the preferred embodiment herein illustrated and described, i.e., primary pole-pieces 16, 17 are disposed in the tank whereas secondary pole pieces 38, 39 are disposed in the lower region thereof. Pole pieces 38, 39 are substantially identical in configuration to pole pieces 16, 17 and are similarly secured to the opposite axial sides of vacuum tank 11. In this connection, shoulders 41, 42 are respectively provided at the outer faces of pole pieces 38, 39 for engaging recesses 43, 44 in the interior side surfaces of the vacuum tank. A compression spring 46 with its ends engaging opposed spot faces 47, 48 respectively formed in the inner faces of pole pieces 38, 39 then urges the pole pieces into intimate contact with the side surfaces of the vacuum tank. The pole tips 49, 51 of pole pieces 38, 39 are, moreover, inclined toward the axis of vacuum tank 11 in the direction of beam motion therealong in the same manner as pole tips 27, 28 of pole pieces 16, 17. The secondary field established between pole pieces 38, 39 is accordingly of similar configuration as the primary field but in transverse spatial opposition thereto.

In order to provide magnetomotive force for establishing the secondary magnetic field between pole pieces 38, 39, a permanent horseshoe magnet 52 is disposed externally of vacuum tank 11 in partial encompassing relation with the mid-length region thereof and in diametric opposition to primary magnet 29. The pole tips of secondary magnet 52 similarly intimately contact the opposite axial sides of vacuum tank 11 and are secured to laterally projecting mounting blocks 53, 54 rigidly attached thereto. Mounting blocks 53, 54 are similar to primary magnet mounting blocks 31, 32 and extend from the mid-length region of the vacuum tank proximate the pole tips of magnet 52 to positions externally opposite pole pieces 38, 39 respectively, in order to provide a preferential flux path thereto.

Jack bolts 56, 57 threadably engaging mounting blocks 53, 54 respectively and jack screws 58, 59 inserted through the pole tips of magnet 52 and secured to the jack bolts are preferably employed as the means for securing the magnet to the mounting blocks. Magnet 52 is thereby rendered translatable with respect to mounting blocks 53, 54 to establish an adjustable length air gap therebetween. The strength of the secondary magnetic field generated within vacuum tank 11 between pole pieces 38, 39 may accordingly be varied by translation of secondary magnet 52 as in the case of primary field strength variation by translation of primary magnet 19.

Although a permanent magnet is employed as secondary magnet 52 of the preferred embodiment of this invention, it will be appreciated that a direct current energized electromagnet may be alternatively employed. In either case, the pole tips of secondary magnet 52 are oriented in polar opposition to the pole tips of primary magnet 29. The secondary magnetic field established transversely be tween poles pieces 38, 39 accordingly is of opposite polarity to the primary field established on the opposite side of the axis of vacuum tank 11 transversely between pole pieces 16, 17. The secondary field thus partially cancels the effects of the primary field in the region of field line overlap near the vacuum tank axis. Such partial cancellation of magnetic fields produces an extremely sharp gradient in magnetic field strength symmetrically across the axis of vacuum tank 11. Such a sharp gradient in field strength together with the spatial distributions of the fields effected by the inclined pole tips 27, 28 and 49, 51 operate to deflect charged particles laterally outward from the vacuum tank axis by varying large amounts dependent upon the lateral positions of the particles relative to the axis.

Accordingly, in operation, the beam difluser of the present invention is coupled by means of vacuum tank inlet section 12 in axial alignment with the beam exit end of a high energy charged particle source, e.g., a

charged particle accelerator. The charged particle beam- 61 (see Figure 5(a)) thus propagates axially throughupper regions of the'vacuum 'vacuum tank 11 and therein encounters the transverse primary heterogeneous field 62 and transverse secondary heterogeneous field 63 of opposite polarity respectively established between pole pieces 16, 17 and 38, 39. Both the primary and secondary fields 62, 63 progressively decrease in strength laterally toward the axis. In the region 64 of substantial field line overlap near the axis, the primary and secondary fields partially cancel each other and establish a steep field strength gradient within such region as well as zero field at the axis. Thus a beam particle passing through the primary and second ary fields at the axis, e.g., particle a, passes undeflected through vacuum tank 11 and axially out of the exit end thereof as depicted in the schematic elevation view of Figure 5 (b). Conversely a beam particle passing through the primary field in a region of high field strength at a point of relatively great lateral displacement from the axis, e.g., particle b disposed in the beam near the periphery thereof, experiences a large magnetic deflection force laterally outward. Moreover, it is to be noted (see Figure 5(b)) that particle b. enters the primary field at a point transversely opposite the upper edges of inclined pole tips 27, 2 8. The primary field thus acts upon particle b ,over the entire axial distance between the upper and lower edges of the pole tips and materially enhances the deflection of the particle. A particle c entering the primary field at a lateral position intermediate particles a and b encounters a field strength of intermediate value substantially less than that encountered by particle b by virtue of the steep field gradient effected across the beam by partial cancellation of the field. It is to be further noted that particlec enters the primary field transversely opposite intermediate points on the inclined surfaces of poles tips 27, 28. The field accordingly acts upon particle 0 over an axial distance given approximately by the distance from the intermediate point on the inclined surface of one of the pole tips to the lower edge surface thereof. Such distance is substantially less than the distance of previous mention over which the field acts upon particle b. As a result of the intermediate value of field strength encountered by particle c and the relatively shorter axial distance over which the field acts, particle c is deflected laterally outward from the beam axis to a position intermediate the axis and thedeflected position of particle b. Similarly, beam particles d and e corresponding in lateral position to particles b and 0 respectively, but on the opposite side of the axis therefrom, encounter the secondary field of opposite polarity and are deflected laterally outward from the beam axis in the same manner as hereinbefore described with respect to particles b and 0.

- In view of the foregoingit will be appreciated that all particles of the beam 61 are continuously deflected laterally outward from the beam axis by amounts which are directly dependent upon the lateral positions of the individual particles in the beam. The beam 61 is thus difiused or spread in the lateral direction in passage through vacuum tank 11 of the beam diffuser of the present invention. Such diifused beam emerging from the exit end of vacuum tank 11 is distributed over a relatively large dosage area 66 of material to be irradiated disposed beneath vacuum tank flange 14. Moreover, the size and shape of dosage area 63' may be readily varied by adjusting jack screws 36, 37 and 58, 59 to responsively appropriately vary the strengths of the primary and secondary heterogeneous magnetic deflection fields estab-.

lished within vacuum tank 11.

While the invention has been disclosed with respecthereinillustrated and described as being in stationary attachment with the Walls of vacuum tank 11, in some-- 7. instances it is desirable that the pole pieces be transversely translatable with respect thereto. The pairs of pole pieces 16, 17 and 38, 39 may then be translated to vary the lengths of the field gaps therebetween with attendant variations being thereby produced in the configurations of the primary and secondary magneticdeflection fields. Various means well known in, the art maybe employed to render the pole pieces transversely translatable. For example, the pole pieces may to lead screws extending bores in the walls of vacuum tank 11. The lead screws may then be rotated to advance and retract the pole pieces in the transverse direction and thereby vary the gaps therebetween. Thus, it is not intended to limit the invention except as defined in the following claims.

What is claimed is:

1. A charged particle beam diffuser comprising means for establishing an evacuated region for traversal by a beam of energetic charged particles, means for generating within said region a primary heterogeneous magnetic field transverse to the axis of beam traversal therethrough, said field progressively increasing in strength laterally outward from the beam axis, said field spatially distributed along the beam axis to be efiective over an axial distance progressively increasing with respect to outward lateral displacement from the axis, and means for generating a secondary heterogeneous magnetic field within said evacuated region transverse to the beam axis, said secondary field increasing in strength laterally outward from the beam axis, said secondary field spatially distributed along the beam axis to be efiective over an axial distance progressively increasing with respect to outward lateral displacement from the axis, said secondary field generated in spatial and polar opposition to said primary field to partially cancel said primary and secondary fields and produce a resultant steep field strength gradient laterally across the beam axis whereby said primary and secondary fields progressively increase sharply in strength and the axial distance over which the fields act upon beam particles progressively increases with respect to lateral displacement of the particles from the beam axis to deflect said beam particles laterally outward by amounts with progressively increase with respect to increasing lateral particle displacement from the beam axis.

2. A charged particle beam diffuser comprising vacuum tank means for establishing an evacuated region for traversal by a beam of energetic charged particles, a pair of transversely spaced pole pieces disposed within said region and laterally displaced to one side of the axis of beam propagation therethrough for generating upon magnetization a primary steady-state heterogeneous magnetic field to deflect charged particles laterally outward from the beam axis by amounts which progressively increase with respect to lateral displacement of the beam particles from the beam axis, a source of magnetomotive force magnetically coupled to said pole pieces to effect generation of said magnetic field therebetween, a second pair of transversely spaced pole pieces disposed within said region and laterally displaced to the opposite side of the beam axis from said first pair of pole pieces for generating upon magnetization a secondary steadystate heterogeneous magnetic field in spatial opposition to said primary field to deflect charged particles laterally outward from the beam axis by amounts which progressively increase with respect to lateral displacement of the beam particles from the beam axis, and a second source of magnetomotive force in polar opposition to said field source and magnetically coupled to said second pair of pole pieces to effect generation of said secondary magnetic field therebetween, said secondary magnetic field being thereby in polar opposition to said primary field and efiecting partial cancellation of magnetic field therebetween to establish a steep field'strength gradient laterally of the beam axis whereby said primary and secondary magnetic fields continuously diffuse said beam parbe individually secured through pressure sealed tapped ticles over a relatively large area laterally of the beam ans.

3. A charged particle beam difiuser as defined by claim 2, further defined by said first and second pairs of pole pieces having shaped pole tips to generate said primary and secondary magnetic fields with spatial distributions along the beam axis each efiective over an axial distance with progressively increases with respect to outward later-al displacement from the beam axis.

4. A charged particle beam difluser as defined by claim 2, further defined by said first and second sources of magnetomotive force beingadjustable to render the strengths of said primary and secondary fields variable whereby the amount of beam difiusion may be varied.

5. A charged particle beam difiuser comprising a vacuum tank having beam inlet means for axially aligned connection to a source of energetic charged particlm and beam exit means axially opposite said inlet means, a pair of transversely spaced pole pieces disposed within said tank between said inlet and outlet means and laterally displaced to one side of the vacuum tank axis, said pole pieces respectively having pole tips inclined toward the axis of said vacuum tank in the direction of said exit means, a magnet magnetically coupled between said pole pieces, a second pair of transversely spaced pole pieces disposed within said tank between said inlet and outlet means and laterally displaced to the opposite side of the vacuum tank axis from said first pair of pole pieces, said second pair of pole pieces respectively having pole tips inclined toward the axis of said vacuum tank in the direction of said exit means, and a second magnet magnetically coupled between said second pair of pole pieces and opposed in polarity to said first magnet whereby a charged particle beam introduced to said inlet means issues from said outlet means enlarged in the lateral direction.

6. A charged particle beam diffuser comprising an axially. elongated rectangular vacuum tank having an axially projecting tubular inlet section for connection to a source of energetic charged particles and a beam outlet disposed axially opposite said inlet section, a pair of transversely spaced pole pieces disposed with the outer sides thereof respectively secured in intimate contact with opposite axial sides of said vacuum tank in the end region thereof proximate said inlet section, said pole pieces laterally displaced to one side of the vacuum tank axis and having pole tips inclined toward the axis in the direction of said outlet, a pair of axially elongated mounting blocks respectively externally secured to said opposite axial sides of said vacuum tank and projecting laterally outward therefrom, said mounting blocks extending axially from the mid-length region of said tank to positions externally opposite said pole pieces, a horseshoe magnet disposed in partial encompassing relation about the mid-length region of said vacuum tank with the inner sides of the magnet in intimate contact with the outer axial sides of said tank and the pole tips of the magnet in end abutment with said mounting block, adjustable fastening means securing said magnet to said mounting blocks and rendering the magnet adjustably translatable with respect to the blocks for establishing an adjustable length air gap therebetween, a second pair of transversely spaced pole pieces disposed with the outer sides thereof respectively secured in intimate contact with said opposite axial sides of the vacuum tank in the end region thereof proximate said beam outlet, said pole pieces laterally displaced to the opposite side of the vacuum tank axis from said first pair of pole pieces and having pole tips inclined in the direction of said outlet, a second pair of axially elongated mounting blocks respectively externally secured to said opposite axial sides of said vacuum tank and projecting laterally outward therefrom, said mounting blocks extending axially from the mid-length region of said tank to positions external- 1y opposite said second pair of pole pieces, a second.

horseshoe magnet disposed in partial encompassing relation about the mid-length region of said vacuum tank in diametric and polar opposition to said first magnet, said second magnet having its opposite inner sides in intimate contact respectively with said opposite axial sides of the vacuum tank and its pole tips in end abutment with said second pair of mounting blocks, and adjustable fastening means securing said second magnet to said second pair of mounting blocks and rendering said second magnet adjustably translatable with respect to the blocks for establishing an adjustable length air gap therebetween whereby a charged particle beam introduced to said inlet section issues from said beam outlet enlarged in the lateral direction by an amount which may be varied by adjustment of said fastening means.

No references cited. 

